from the suitcase and choose a volunteer to come up front.
the student one end of the slinky and ask her (him) to
hold this end steady at mid-chest level. Stand 4 feet away and
begin to vibrate your end up and down and in a circle. You should
be able to create a standing wave with a few nodes, but the
plane of vibration should not be well defined.
that, in addition to color, light has a wave nature. The slinky
represents a light wave. This motion represents unpolarized
light - light without a preferred vibration direction. [For
simplicity, we ignore circularly polarized light.]
the circular motion and vibrate only vertically. State that
light is polarized when it vibrates in one direction
- vertical or (switch hand motion) horizontal. [Horizontal
motion is a bit harder to maintain while speaking and you might
go back to the vertical motion]
linearly polarized light as light whose vibration direction
is in a plane. [Here you might use the
overhead transparency from NASA that shows the spectrum
from radio to gamma-rays, with low frequency, long wavelength
and high frequency, short wavelength radition. The photos at
the bottom of the transparency show what the Milky Way looks
like through different telescopes that look at differ-
ent frequencies or wavelengths of optical radiation.]
the slinky away.
the teacher hand out the Magic Stripes
Theme Packet, but ask the students not
to open them yet. When everyone has their own packet,
proceed as follows:
the students to remove everything from the packet and place the
5 items on their workspace. (See Figure 3.) Have them check that
they have two dark pieces of plastic, one transparent piece of
plastic with writing on it, a fork, and a plastic vial.
your two large pieces of linear sheet polarizer and hold
them up, one in each hand. Combine them about one foot in front
of your face with the transmission axes parallel. You should be
able to see the students, and they should be able to see you.
Ask them to make a sandwich in the same way with their dark pieces
the oohs and ahs, reveal that each dark
plastic has a secret code on its surface in the form of small
lines. Combining the plastics so that the lines are parallel makes
it possible to see though them. Combining with lines perpendicular,
or crossed, blocks the light. Identify the plastics
as linear polarizers. One plastic held up to the room lights causes
the unpolarized white light to become linearly polarized. Once
the light is linearly polarized it vibrates in one plane, and
it is either transmitted or absorbed by the second plastic polarizer.
Polarized sunglasses are made of this plastic.
the teacher turn off the lights. Place the large polarizers on
the over head projector and demonstrate how they polarize and
extinguish the white light from the projector. Separate the crossed
polarizers at four corners with the plastic cups
to make a polariscope, place the plasticware inside, stand
back, and enjoy the excitement.
where the color comes from. [You might get
some correct answers.]
that stresses inside of transparent materials degrade the quality
of linearly polarized light coming through the polariscope, causing
various colors to show up. With a polariscope, geologists identify
certain crystals and mineral structures. Civil engineers examine
stresses inside structures made out of transparent plastic, to
understand how to build them better. Photonics technicians evaluate
the quality of laser glasses and laser crystals with polariscopes.
the room lights turned back on. With the items from your packet,
show the students how to make a polariscope in one hand. Ask them
to find the colored stripes in the clear sheet of plastic from
their packet. [While looking through the
crossed polarizer sandwich at the overhead lights, they must insert
the clear plastic between the polarizers.]
the students to evaluate the internal stresses in the plastic
vial and the fork. By squeezing on the tines of the fork, the
students may be able to induce and visualize additional stresses.
everyone put all items back in the packets. Suggest that, once
home, the students may demonstrate the magic stripes trick to
their families, since they know the secret polarizer code and
how to construct a polariscope.
3. Contents of the Magic Stripes Theme Packet, including supplies
for making a polariscope.
by Stephen D. Jacobs, Rebecca L. Coppens and Christine Andrews-Angelo
December 24, 2001
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